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A group of glycoproteins that are embedded in the surface of the membranes of certain cells and which have one or more immunoglobulin domains. Each domain is a chain of about 100 amino acids that folds back and forth upon itself to form a sandwich of two pleated sheets linked by a disulfide bond. Included in the superfamily are the immunoglobulins (q.v.) with up to 12 domains per molecule, the T cell receptors (q.v.), and the MHC receptors (q.v.), each with two domains per molecule, and the CD4 and CD8 receptors (q.v.) with four domains and a single domain, respectively. The genes that encode these proteins are assumed to have evolved from a common ancestral gene over a period of hundreds of millions of years.